Periodic solutions for second order delay Duffing
equation via critical point theory

Jingrui Zhang, Yi Cheng, Changqin Yuan
Abstract:
This article concerns the periodic problem of second order
delay Duffing equation with cross resonance condition.
Using critical point theory and homotopy methods, we obtain
sufficient conditions for the existence of periodic solutions.
Also, we present an example to illustrate our results.

This article was withdrawn by the authors on July 21, 2013.
"We found some fatal errors in
the proof of section 3, so that many structural proofs are wrong.
According to academic rigor, we think it is not suitable for publication.
So we decided to withdraw our paper"